Zero crossing switches, zero crossing solid state relays (ZCSSRs) in particular, were designed heretofore to be an inexpensive, effective solution for interface applications between low current DC control circuits and AC power loads. The turn ON/OFF at AC zero crossing feature was expected to minimize current surges which consequently reduces the generation of EMI. Although EMI is always a problem, it becomes a serious problem when "machine operation" is affected or the EMI generated by a "machine" exceeds FCC Regulations. The prior art teaches that ZCSSRs can be turned ON or OFF with no need to consider the ZCSSRs turn ON/OFF signal specifications which will be described hereinafter in conjunction with FIGS. 1 and 2. Heretofore, it has been assumed that if the turn ON/OFF signal specifications were not met at the first zero cross point, the ZCSSR would just wait for the next zero cross point to turn ON/OFF without causing any problems. However, as will be explained in more detail below, Applicants have discovered that the use of random ZCSSR turn ON/OFF command signals relative to the AC line frequency tends to produce excessive conducted EMI. As will also be explained, in detail, synchronizing the ZCSSR ON/OFF command signals with the AC line frequency in accordance with the present invention, such that their ON/OFF signal specifications are never violated, will result in a significant reduction of the conducted EMI levels.
Referring specifically to FIG. 1, the zero crossing switch (ZCS) 10, for example a zero crossing solid state relay, is shown connected in line between a zero crossing AC voltage source 11 and an AC driven load for closing and opening the AC load line in order to connect the AC voltage to and disconnect it from the load. The zero crossing switch is shown being controlled in accordance with the prior art by externally produced command signals 12 which will be described hereinafter in conjunction with FIGS. 3A-3D.
In the meantime, the zero crossing AC voltage used to drive the load illustrated in FIG. 1 is graphically depicted in FIG. 2 at 14. As seen in this latter figure, AC voltage 14 has a constant peak-to-peak amplitude and successive zero crossing points ZC. In the discussion of zero crossing solid state relays above, mention was made of the relays' ON/OFF specifications. As stated there, heretofore it has been assumed that if the turn ON/OFF signal specifications were not met at one zero crossing point, the ZCSSR would just wait for the next zero crossing point to turn ON/OFF without causing any harm. The relays' signal or performance specification requires that an ON signal be present in a predetermined period of time, for example 0.4 milliseconds, before the AC voltages zero crossing point ZC and continue through ZC for the relay to turn ON. It also specifically requires that the OFF signal be present for a predetermined period of time, for example 0.35 milliseconds, before ZC and continue through ZC for the relay to turn off. While most relay performance specifications do not address any required time period for the ON or OFF signal to remain present after ZC, applicants have found that, as a practical matter, such required periods exist. This performance window is indicated by dotted lines at PW in FIG. 2. For purposes of simplicity, only one of the windows, for example the ON window, is illustrated. Thus, in accordance with prior art practice, in order to turn ON zero crossing switch 10 illustrated in FIG. 1 at its zero crossing point ZC, an ON command signal 12 must be present at the beginning of performance window PW and continue throughout the duration of the window.
Referring to FIGS. 3A-3D, a series of externally produced command signals 12A-12D are illustrated. In each case, the negative pulses correspond to ON command signals and the positive pulses correspond to OFF signals. Each series of command signals is shown synchronized (time wise) with AC voltage 14. For purposes of discussions, let it be assumed that the intent of each series of command signals illustrated in FIGS. 3A-3D is to turn on zero crossing switch 10 and thereby connect the AC voltage to its associated load. With particular reference to FIG. 3A, note that the only ON pulse of series 12A falling within performance window PW is the shaded negative pulse. Note further that this later pulse begins well after the beginning of the window and, hence, will not turn ON switch 10, as per its performance specification. Therefore, according to the prior art, the switch 10 waits for another ON pulse and still another one, if necessary, until one such pulse falls entirely within window PW. In the meantime, it should be still further noted that, in the case of series 12A, a transition is made from an OFF pulse to the shaded ON pulse within the performance window. This can happen many times before the performance specification is met and the switch is turned ON. Prior art practice has apparently ignored these transitions as being insignificant. Applicants, on the other hand, have found that these transitions within the performance window can and often do result in significant current surges resulting in significant electromagnetic interference. The signal series 12D shown in FIG. 3D suffers from the same problem as signal series 12A. In FIG. 3B, the shaded negative ON pulse beings prior to performance window PW but transitions to the OFF state within the window. Only signal series 12C in FIG. 3C includes a valid ON pulse (the shaded negative Pulse).